I would agree that a food change is in order. Go grain-free if you can. My guy loves Taste of the Wild and he tolerates it very well. You should see a huge difference once he starts getting the fuel he needs to power that little 2.9lb body!
I might add, Petco and large chains don't carry this or several other high quality foods that area available at smaller "health" food pet stores. And once you do find a suitable store, many of them carry small sample sizes of the foods that you could try before buying. Just pick one or two though so your baby doesn't get an upset tummy from switching too much.